In order to evaluate the nature of underground formations surrounding a borehole, it is often desirable to obtain samples of formation fluids from various specific locations in a borehole. Tools have been developed which allow several samples to be taken from the formation in a single logging run. Examples of such tools can be found in U.S. Pat. Nos. 3,780,575 and 3,859,851.
Schlumberger's RFT and MDT tools represent two specific versions of sampling tools. In particular, the MDT tool includes a fluid analysis module to allow analysis of fluids sampled by the tool. FIG. 1 illustrates a schematic diagram of such a downhole tool 10 for testing earth formations and analyzing the composition of fluids from the formation. The downhole tool 10 is suspended in a borehole 12 from a logging cable 15 that is connected in a conventional fashion to a surface system 18 incorporating appropriate electronics and processing systems for control of the tool and analysis of signals received from the downhole tool 10. The downhole tool 10 includes an elongated body 19 which encloses a downhole portion of a tool control system 16. The elongated body 19 also carries a selectively extendible fluid admitting assembly 20 (for example as shown in the '575 and '851 patents referenced above, and as described in U.S. Pat. No. 4,860,581, incorporated herein by reference) and a selectively extendible anchoring member 21, which are respectively arranged on opposite sides of the elongated body 19. The fluid admitting or withdrawal assembly 20 is equipped for selectively sealing off or isolating portions of the wall of the borehole 12 such that pressure or fluid communication with the adjacent earth formation is established. A fluid analysis module 25 is also included within the elongated tool body 19, through which the obtained fluid flows. The fluid can then be expelled through a port (not shown) back into the borehole, or it can be sent to one or more sample chambers 22, 23 for recovery at the surface. Control of the fluid withdrawal assembly, the fluid analysis section and the flow path to the sample chambers is maintained by the electrical control systems 16, 18.
An optical fluid analyzer (OFA), which may be located in the fluid analysis module 25, may identify the fluids in the flow stream and quantify the oil and water content. U.S. Pat. No. 4,994,671 (incorporated herein by reference) describes a borehole apparatus which includes a testing chamber, a light source, a spectral detector, a database, and a processor. Fluids drawn from the formation into the testing chamber are analyzed by directing the light at the fluids, detecting the spectrum of the transmitted and/or backscattered light, and processing the information (based on information in the database relating to different spectra), in order to characterize the formation fluids.
In addition, U.S. Pat. Nos. 5,167,149, and 5,201,220 (both incorporated by reference herein) describe apparatus for estimating the quantity of gas present in a fluid stream. A prism is attached to a window in the fluid stream and light is directed through the prism to the window. Light reflected from the window/fluid flow interface at certain specific angles is detected and analyzed to indicate the presence of gas in the fluid flow.
As set forth in U.S. Pat. No. 5,266,800 (incorporated herein by reference), monitoring optical absorption spectrum of fluid samples obtained over time may allow one to determine when formation fluids, rather than mud filtrates, are flowing into the fluid analysis module 25. Further, as described in U.S. Pat. No. 5,331,156 to Hines, by making optical density (OD) measurements of the fluid stream at certain predetermined energies, oil and water fractions of a two-phase fluid stream may be quantified.
In addition or alternative to the optical fluid analyses described above, the in situ measurement of other formation fluid properties may be desired. For example, for fluid samples that will be returned to the surface for further analysis, it may be important to precisely measure the density, viscosity, temperature, and pressure of a fluid sample downhole. During transportation of a sample bottle from a field location to a laboratory, the fluid properties may change due to differences in pressure and temperature. The in situ conditions must be precisely known in order to duplicate the conditions in a laboratory for full and accurate sample analysis. However, current downhole analysis equipment associated with fluid sample characteristic measurements is quite large and requires significant power resources. Moreover, space and power are at a premium in a downhole tool, and therefore the smaller and more power efficient the equipment, the better.
The present invention is directed to improving, or at least reducing the effects of, one or more of the problems identified above.